I think as Snooze said, in the private sector it's all about how much you can earn for the company you work for, or inversely how much you can avoid them losing.
If for instance a car salesman pulled in £3m to the business maybe you'd pay him 10% + a nominal base salary.
Don't pay enough and someone will offer 11% + a nominal base salary and they'd move - you'd have a net loss of £3m.
I don't think many are altruistic enough to say "no, I'm ok on the raise, I earn enough to get by thanks."
Most people get market worth - not much more, not much less and salary is always applicable to the market you work in.
But to give an example - a quantative analyst working for say a school as a physicist/maths teacher will get a school teacher's pay, where as working as a quantative analyst doing risk assessment for financials would probably pay multiples of that. Same skills, different market.